Degenerative disc disease is typically caused by a loss of disc space height, leading to a narrowing of the neural foramen and subsequent neural compression, and causing back and radicular pain. Instability of the posterior elements can lead to conditions such as spondylolisthesis or spinal stenosis. In the case of spondylolisthesis, a vertebral body slips forward in relation to an adjacent vertebrae. This movement of the vertebral body narrows the foramen and results in painful pressure on the nerve roots. In the case of spinal stenosis, the spinal canal narrows and compresses the spinal cord and nerves.
Degenerative disc disease may often be resolved through a spinal fusion procedure using an interbody implant (one which is implanted between the bodies of two adjacent vertebrae). Interbody implants have been used since the mid 1930s to aid in spinal fusion. Such interbody implants may be formed from titanium, carbon fiber, allograft, or other suitable material including, but not limited to, biocompatible materials such as the Paek family of polymers. Implantation of a substitute graft is designed to reestablish normal disc height, provide immediate stability to the motion segment, and provide a matrix for fusion of the implant with the patient's natural bone structures. Bone tissue is capable of regeneration and will grow if adequate space is provided. Therefore, when the patient's bone grows into the implant device, the fusion becomes solid and movement is eliminated at that level. Additionally, non fusion techniques have been introduced to aid in motion preservation of the disc space. These are known as total disc replacements (TDR). A mechanical device is implanted into the disc space much like a fusion device but its goal is to prevent fusion of the interbody space and preserve movement in the motion segment. These devices are made from various materials similar to fusion devices. Also, nucleus replacement devices have been introduced to preserve the annulus and maintain disc height. These devices are small and can be introduced to the disc space like fusion devices but through smaller incisions.
Typically, an open implant device is filled with a graft material and placed inside the disc space. Such graft material may come from the patient's own body. Alternatively, the graft material may be any suitable artificial, synthetic, or natural substitute. Once the implant containing the graft material is properly placed in the disc space, a biological reaction is triggered, which results in bone growth. Over time, as the patient's native bone begins to grow, the natural bone replaces the graft material, resulting in new bone located in the target region of the spine.
In order for the patient's native bone to grow appropriately, it is important that the implant remain properly in place in the interbody space. Typical implants employ many different approaches to implant contours in an effort to minimize post-operative migration of the implant. These approaches typically provide a tooth geometry and alignment designed to resist movement and migration along the anterior-posterior axis. This single axis implant geometry combined with the unnatural, non-anatomical implant shape typically also results in contact between the implant and vertebral endplates at discreet points. These point contacts may result in substantial discomfort and pain for the patient and contribute very little to the prevention of post-operative migration of the implant. In fact, the non-optimal contact between the implant and the vertebral walls increases the likelihood of post-operative movement of the implant, decreases the chance for fusion, and increases Pseudoarthrodesis.
The interbody space for lumbar surgery has always challenged surgeons trying to access the space to achieve arthrodesis. Multiple surgical methods have been employed to place the interbody implant into the disc space: a posterior approach (posterior lumber interbody fusion—PLIF), a transforaminal approach (transforaminal lumbar interbody fusion—TLIF), an anterior approach (anterior lumbar interbody fusion—ALIF) or a direct lateral approach (extreme lateral interbody fusion—XLIF).
Proper distraction during a PLIF procedure must be achieved to gain compression of the implant through ligamentous taxis. Proper distraction allows natural compression across the disc space via the annulus and other posterior elements as well as the anterior longitudinal ligament. This compression delivered to the implant helps stabilize the implant, which prevents expulsion, and keeps the grafting material under stress, thus promoting faster fusion and bone healing. Existing techniques for reaching the interbody space from a posterior approach include the use of Cloward dowels, threaded cages, impacted cages and impacted allografts. All of these techniques have limitations as well as complications, as they involve extensive nerve root retraction as well as destabilization through destruction of bony and ligamentous structures.
TLIF involves the removal of one facet joint, usually on the more diseased or symptomatic side of the spine. PLIF is usually performed bilaterally, removing a portion (if not all) of each of the facet joints. Removal of the entire facet joint improves visualization into the disc space, allowing removal of more disc material and insertion of a larger implant. The transforaminal approach limits the nerve root injuries associated with the PLIF procedure because the disc space and spinal canal is approached from one side of the intervertebral space. This allows the surgeon to operate with minimal stretching of nerve roots. Various banana-shaped implants have been designed to be impacted across the disc space to achieve arthrodesis. Although longer, straight implants have been placed across the disc space with some success, the lordotic angle of the spine is harder to properly match with these straight implants. The banana-shaped implant helps maintain proper lordosis when it is placed in the anterior third of the disc space. Despite the benefits of the TLIF procedure, TLIF still suffers from limitations involving bony and soft tissue destruction and bilateral pathology.
ALIF is utilized to avoid the posterior structures of the spine. However, the anterior approach (from the patient's abdomen) to the disc space also presents challenges and limitations because of the potential of vascular injuries. In addition, not all of the lumbar spinal segments can be reached from an anterior incision without potential complications. Retroperitoneal approaches have helped eliminate some of the vascular injuries, but the potential still exists. It is known in the art that revision surgery is greatly complicated by scarring from the initial procedure.
XLIF was devised in an attempt to avoid the complications associated with the posterior and anterior approaches to the spine. This technique provides an additional way to access the interbody space for fusion as well as for motion preservation procedures. XLIF is useful for lumbar fusions from L1-L5 and preserves the entire posterior envelope of the spine. The XLIF procedure can also be performed at levels above the lumbar spine in the thoracic region. XLIF is minimally invasive in that it does not involve cutting muscle tissue. While there is potential for nerve injury (though limited by using nerve monitoring equipment) and psoas muscle irritation, the muscles are spared through dilation instruments. Once the disc space is exposed, complete discectomy can be performed to prepare the fusion bed. Since the XLIF procedure avoids anterior entry, vascular structures are not compromised or scarred, eliminating possible complications in following salvage procedures. Another drawback of existing systems and techniques for XLIF procedures is that implants are usually undersized from a medial-lateral and anterior-posterior approach. When the implant is undersized, and not resting on the cortical edges of the vertebral bodies, they can piston through or subside into the softer, interior portions of the vertebral bodies. This can occur with or without endplate sparing techniques.
Each approach has its limitations as well as advantages. From a posterior (PLIF) or transforaminal (TLIF) approach, the individual implants are usually smaller because of the neural structures that prevent access to the total disc space. From an anterior (ALIF) or far lateral (XLIF) approach, the implants are usually quite larger and a fuller, more complete discectomy can be performed without the limitations of retracting neural structures. Thus, larger implants can be utilized that hold more graft material. Regardless of the approach, each implant inserted into the disc space will hold a volume of graft material with the intent of triggering the bone growth biological response.
Although the ALIF, PLIF, TLIF and XLIF approaches are discussed above, other approaches have also been developed. These and future approaches may be implemented in connection with the present invention. In addition to the fusion devices described herein, the same concepts of tooth geometry and implant angles (both symmetrical and asymmetrical) can be applied to non-fusion devices. Non fusion devices may include total disc replacement implants implanted singly or in pairs from anterior, lateral, transforaminal, posterior, anteriolateral, laparasccopic or any comparable approach. The design is also applicable to nucleus replacement devices for the disc space.
In existing systems, regardless of the surgical approach, the implants are not designed to fit the anatomical contours of the spinal cavity. Thus, the implants are subject to post-operative migration and repulsions, resulting in significant pain and discomfort for the patient. What is needed is a spinal implant designed to fit the anatomical contours of the spinal cavity that prevents movement and migration of the implant.